Chapter 15 Event-Driven Programming and Animations

Motivations Suppose you want to write a GUI program that lets the user enter a loan amount, annual interest rate, and number of years and click the Compute Payment button to obtain the monthly payment and total payment. How do you accomplish the task? You have to use event-driven programming to write the code to respond to the button-clicking event. LoanCalculator Run

Objectives To get a taste of event-driven programming (§15.1). To describe events, event sources, and event classes (§15.2). To define handler classes, register handler objects with the source object, and write the code to handle events (§15.3). To define handler classes using inner classes (§15.4). To define handler classes using anonymous inner classes (§15.5). To simplify event handling using lambda expressions (§15.6). To develop a GUI application for a loan calculator (§15.7). To write programs to deal with MouseEvents (§15.8). To write programs to deal with KeyEvents (§15.9). To create listeners for processing a value change in an observable object (§15.10). To use the Animation, PathTransition, FadeTransition, and Timeline classes to develop animations (§15.11). To develop an animation for simulating a bouncing ball (§15.12).

Procedural vs. Event-Driven Programming Procedural programming is executed in procedural order. In event-driven programming, code is executed upon activation of events.

Taste of Event-Driven Programming The example displays a button in the frame. A message is displayed on the console when a button is clicked. HandleEvent Run

Handling GUI Events Source object (e.g., button) Listener object contains a method for processing the event. 6

1. Start from the main method to create a window and display it animation Trace Execution public class HandleEvent extends Application { public void start(Stage primaryStage) { … OKHandlerClass handler1 = new OKHandlerClass(); btOK.setOnAction(handler1); CancelHandlerClass handler2 = new CancelHandlerClass(); btCancel.setOnAction(handler2); primaryStage.show(); // Display the stage } class OKHandlerClass implements EventHandler<ActionEvent> { @Override public void handle(ActionEvent e) { System.out.println("OK button clicked"); 1. Start from the main method to create a window and display it 7

Trace Execution public class HandleEvent extends Application { animation Trace Execution public class HandleEvent extends Application { public void start(Stage primaryStage) { … OKHandlerClass handler1 = new OKHandlerClass(); btOK.setOnAction(handler1); CancelHandlerClass handler2 = new CancelHandlerClass(); btCancel.setOnAction(handler2); primaryStage.show(); // Display the stage } class OKHandlerClass implements EventHandler<ActionEvent> { @Override public void handle(ActionEvent e) { System.out.println("OK button clicked"); 2. Click OK 8

3. Click OK. The JVM invokes the listener’s handle method animation Trace Execution public class HandleEvent extends Application { public void start(Stage primaryStage) { … OKHandlerClass handler1 = new OKHandlerClass(); btOK.setOnAction(handler1); CancelHandlerClass handler2 = new CancelHandlerClass(); btCancel.setOnAction(handler2); primaryStage.show(); // Display the stage } class OKHandlerClass implements EventHandler<ActionEvent> { @Override public void handle(ActionEvent e) { System.out.println("OK button clicked"); 3. Click OK. The JVM invokes the listener’s handle method 9

Events An event can be defined as a type of signal to the program that something has happened. The event is generated by external user actions such as mouse movements, mouse clicks, or keystrokes.

Event Classes

Event Information An event object contains whatever properties are pertinent to the event. You can identify the source object of the event using the getSource() instance method in the EventObject class. The subclasses of EventObject deal with special types of events, such as button actions, window events, component events, mouse movements, and keystrokes. Table 16.1 lists external user actions, source objects, and event types generated.

Selected User Actions and Handlers

The Delegation Model

The Delegation Model: Example Button btOK = new Button("OK"); OKHandlerClass handler = new OKHandlerClass(); btOK.setOnAction(handler);

Example: First Version for ControlCircle (no listeners) Now let us consider to write a program that uses two buttons to control the size of a circle. ControlCircleWithoutEventHandling Run

Example: Second Version for ControlCircle (with listener for Enlarge) Now let us consider to write a program that uses two buttons to control the size of a circle. ControlCircle Run

Inner Class Listeners A listener class is designed specifically to create a listener object for a GUI component (e.g., a button). It will not be shared by other applications. So, it is appropriate to define the listener class inside the frame class as an inner class.

Inner Classes Inner class: A class is a member of another class. Advantages: In some applications, you can use an inner class to make programs simple. An inner class can reference the data and methods defined in the outer class in which it nests, so you do not need to pass the reference of the outer class to the constructor of the inner class. ShowInnerClass

Inner Classes, cont.

Inner Classes (cont.) Inner classes can make programs simple and concise. An inner class supports the work of its containing outer class and is compiled into a class named OuterClassName$InnerClassName.class. For example, the inner class InnerClass in OuterClass is compiled into OuterClass$InnerClass.class.

Inner Classes (cont.) An inner class can be declared public, protected, or private subject to the same visibility rules applied to a member of the class. An inner class can be declared static. A static inner class can be accessed using the outer class name. A static inner class cannot access nonstatic members of the outer class

Anonymous Inner Classes An anonymous inner class must always extend a superclass or implement an interface, but it cannot have an explicit extends or implements clause. An anonymous inner class must implement all the abstract methods in the superclass or in the interface. An anonymous inner class always uses the no-arg constructor from its superclass to create an instance. If an anonymous inner class implements an interface, the constructor is Object(). An anonymous inner class is compiled into a class named OuterClassName$n.class. For example, if the outer class Test has two anonymous inner classes, these two classes are compiled into Test$1.class and Test$2.class.

Anonymous Inner Classes (cont.) Inner class listeners can be shortened using anonymous inner classes. An anonymous inner class is an inner class without a name. It combines declaring an inner class and creating an instance of the class in one step. An anonymous inner class is declared as follows: new SuperClassName/InterfaceName() { // Implement or override methods in superclass or interface // Other methods if necessary }

Anonymous Inner Classes (cont.) AnonymousHandlerDemo Run

Simplifying Event Handing Using Lambda Expressions Lambda expression is a new feature in Java 8. Lambda expressions can be viewed as an anonymous method with a concise syntax. For example, the following code in (a) can be greatly simplified using a lambda expression in (b) in three lines.

Basic Syntax for a Lambda Expression The basic syntax for a lambda expression is either (type1 param1, type2 param2, ...) -> expression or (type1 param1, type2 param2, ...) -> { statements; } The data type for a parameter may be explicitly declared or implicitly inferred by the compiler. The parentheses can be omitted if there is only one parameter without an explicit data type.

Single Abstract Method Interface (SAM) The statements in the lambda expression is all for that method. If it contains multiple methods, the compiler will not be able to compile the lambda expression. So, for the compiler to understand lambda expressions, the interface must contain exactly one abstract method. Such an interface is known as a functional interface, or a Single Abstract Method (SAM) interface. AnonymousHandlerDemo Run

Problem: Loan Calculator Run

MouseEvent MouseEventDemo Run

The KeyEvent Class MouseEventDemo Run

The KeyCode Constants

Example: Control Circle with Mouse and Key Run

Listeners for Observable Objects You can add a listener to process a value change in an observable object. An instance of Observable is known as an observable object, which contains the addListener(InvalidationListener listener) method for adding a listener. Once the value is changed in the property, a listener is notified. The listener class should implement the InvalidationListener interface, which uses the invalidated(Observable o) method to handle the property value change. Every binding property is an instance of Observable. ObservablePropertyDemo Run DisplayResizableClock Run

Animation JavaFX provides the Animation class with the core functionality for all animations.

PathTransition PathTransitionDemo Run FlagRisingAnimation Run

FadeTransition The FadeTransition class animates the change of the opacity in a node over a given time. FadeTransitionDemo Run

Timeline PathTransition and FadeTransition define specialized animations. The Timeline class can be used to program any animation using one or more KeyFrames. Each KeyFrame is executed sequentially at a specified time interval. Timeline inherits from Animation. TimelineDemo Run

Clock Animation ClockAnimation Run

Case Study: Bouncing Ball BallPane BounceBallControl Run